The increasing densification of the elements of a semiconductor device in recent years has led to a use for an increased densification of the wiring in a substrate on which these elements are mounted. Accordingly, it is now necessary to greatly reduce the size of the vias used for interphasic electric connections.
As the conventional methods of forming vias, there are known a method in which holes are bored in a green sheet with a drill, punch or laser and a paste filled into the thus-bored holes by screen printing; a method in which holes are bored in a green sheet covered with a carrier film, and a paste or powder filled into the thus-bored holes through the carrier film; and a method of forming vias by filling metallic balls directly into a green sheet.
The above methods of filling a paste or powder, however, have the following defects. When a paste is filled into the holes of a substrate or green sheet, organic components contained in the paste are decomposed and volatilized when the substrate or green sheet is calcined, and thus an insufficiently filled state of the vias occurs due to the volume loss of the paste or powder. Even when powder having a small volume loss is filled into holes for making vias, problems arise in that it is difficult to fill the powder into minute holes for vias and that the operating efficiency is lowered. These problems result in the formation of gaps at the interface between a conductor and substrate around the circumference of a via. Furthermore, a weak adhesion between a conductor and a substrate is a problem to be solved, and ideally these problems can be solved at the same time.